COMPOSITION OF SOILS. 



117 



exists in minute quantities in soils; the proportion, according to John- 

 ston, varying from 0'16 to 0"84 in 100 parts, while the insoluble 

 siliceous matter varies from 60'47 to 83'31 in 100 parts. Wiegman 

 and Polstorf found that plants took up the silica from a soil composed 

 entirely of quartz sand, from which every thing organic and soluble 

 had been removed. The following table shows the plants which ger- 

 minated, the height to which they grew previously to being analysed, 

 the quantity of silica they contained when planted, and the increase : 



236. Alumina exists abundantly in clayey soils, but it does not enter 

 largely into the composition of plants. It has the power of absorbing 

 ammonia, and may prove beneficial in this way. 



237. Lime is an essential ingredient in all fertile soils. In 1000 

 Ibs. of such soil, there are, according to Johnston, 56 Ibs of lime; 

 Avhile barren soil contains only 4 Ibs. The presence of phosphoric 

 acid in soils, in the form of phosphates of potass, soda, and lime, is 

 essential for the production of certain azotised compounds in plants ; 

 and sulphuric acid, similarly combined, is required for the formation 

 of others. 



238. A rough way of estimating the general nature of a soil, is thus 

 given by professor Johnston : 



1. Weigh a given portion of soil, heat it and dry it. The loss is water. 



2. Burn what remains. The loss is chiefly vegetable matter. 



3. Add muriatic acid to residue, and thus the quantity of lime may be 



determined. 



4. Wash a fresh portion of soil to determine the quantity of insoluble 



siliceous sand. 



Such an analysis, however, is by no means sufficient for the pur- 

 poses of the farmer. 



239. The chemical composition of a plant being known, conclusions 

 can be drawn as to the soil most suitable for its growth. This is a 

 matter of great importance both to the farmer and to the planter. In 

 order that a plant may thrive, even in a suitable soil, exposure and 

 altitude must also be taken into account. It is only by attention to 

 these particulars that agricultural and foresting operations can be 

 successfuL As regards trees, the following practical observations are 

 given as an illustration of what has been stated. The Scotch Fir 



